REFACTOR: rewrite CMakeLists.txt for better inlcude and reuse

The rewrite uses more cmake build-in automatisms and build-in generates
variables to allow better generic reuse.
* cmake  files are installed to
``` <install_prefix>/lib/cmake/nlohmann_json/ ``` for best support on
most systems
* include path is set to ``` include ```  for usage as ``` #include
<nlohmann/json.hpp> ```

📝 updated documentation

🔀 merge branch 'release/2.1.0' into develop

🔀 merge branch 'release/2.0.10' into develop

🔀 Merge branch 'release/2.0.9' into develop

🔖 Merge branch 'release/2.0.8' into develop

Merge branch 'release/2.0.7' into develop

Merge branch 'release/2.0.6' into develop

Merge branch 'release/2.0.5' into develop

Travis updated their Xcode